Determining which phase has faulted, or which phases have faulted, will hereafter, in accordance with the terminology used within this technical field, be referred to as "phase selection".
A number of different fault types occur in a power network, for which it is desired to learn as quickly as possible in which phase or phases a fault has occurred. The reason for this is that a faulted phase/faulted phases is/are to be disconnected from the supply sources to prevent dangerous situations from arising.
In relatively simple networks and in relatively simple contexts a phase selection can be made, in the event of a fault, by determining by means of phase-current measuring members that a phase current exceeds a pre-set value.
An obvious and simple method, in principle, is to use, as phase selection determining criterion, phase current changes exceeding a certain value related to nominal phase currents. In U.S. Pat. No. 3,956,671, such a method is described which is otherwise based on directional wave detectors for each phase and which comprises a phase selector for single-phase tripping of circuit breakers for a faulted phase and for three-phase tripping of the circuit breakers of all the phases when faults occur on two or three phases.
In another method for phase selection, the voltage reduction of the phases involved, which a fault generally results in, is also utilized in addition to the phase currents. In principle, this comprises using a voltage-dependent overcurrent relay or, as it is called within this technical field, an underimpedance relay. Such relays are described in a number of variants, for example as in ASEA Information RK 556-300 E, November 1974, "Impedance Relay Type RXZF 2" and RK 556-301 E, February 1979, "Three phase impedance relay type RXZK". These relays are activated when an impedance, calculated with measured voltages and currents, lies within an operating range, specific to the relay and defined in an R-X diagram. The methodology in this connection is somewhat different depending on whether the fault is a single-phase or a two-phase fault. Phase selection characteristics of a 3-phase power network, with the aid of underimpedance protection, is also clear from "Schutztechnik in Elektroenergiesystemen" by H. Ungrad, W. Winkler and A. Wiszniewski, Springer-Verlag, published 1991, page 117 and FIG. 6.22.
U.S. Pat. No. 4,864,453 describes a method for selective phase selection in case of faults in distribution systems with double transmission lines between two stations. The method is based on Fourier parameter estimation of phase currents and phase voltages. With the aid of these as well as the residuals of the signals, it is first determined whether an abrupt event has taken place, after which it can be determined, via logical decisions, whether a fault has occurred between the stations as well as which phase or phases has or have faulted.
When a fault occurs in a power networK, this normally results in the network becoming unsymmetrically loaded. Methods for phase selection determination, based on the use of symmetrical components, have therefore often been employed. It is clear, inter alia from GE Application and Setting Guide, 1977, section 4, that the ratio of negative-sequence current I2 to zero-sequence current I0 for both single-pole and three-pole phase selectors is utilized. For single-pole phase selectors the phase position for the symmetrical currents in each phase is compared, and a time-limit is imposed on the comparison means to allow an output signal for a coincidence period corresponding to +/-60.degree.. The disadvantage of using single-pole phase selectors according to this principle is that in the case of two-phase ground faults this method tends to select the faultless phase as the faulted phase. It is therefore necessary to have a three-pole phase selector which covers every conceivable multi-phase fault. The same GE publication also describes a three-pole phase selector which uses the same tripping principle as the single-phase one but where also an additional number of criteria are stated.
In an article entitled "Progress in the Protection of Series-Compensated Lines and in the Determination of Very High Earth-Fault Resistances" in Brown Boveri Rev., 2-81, pages 102/103, a phase selector is also described. The starting point for selecting the correct phase are the zero-sequence current I0 and the negative-sequence voltage U2 for the phase on which a ground fault has occurred. Since the phase position for this voltage is approximately equal to the phase position of the zero-sequence voltage U0 function is obtained in the same way as with directional ground fault relays. By using, in addition, ground fault directional relays which are based on the zero-sequence components, it is possible to determine whether a fault is a ground fault or a fault between the phases. If it is a question of a two-phase ground fault, the start relays of the distance relays are activated, and with the aid of a logic circuit the faultless phase is prevented from being selected.
In EP-B-0 276 181 a phase selection method is described which is based on different linear connections between the above-mentioned symmetrical components. The device comprises, inter alia, six filters and three phase comparators.
As will become apparent from the following description of the invention, the present invention will also be based on symmetrical components. Contrary to the processes mentioned above, where symmetrical components obtained with the aid of conventional RLC filters have been used, a discrete-time numerical technique will, however, be used for the determination. Such a method is described in "Microprocessor-implemented digital filters for the calculation of symmetrical components" by A. J. Degens in IEE PROC., Vol. 129, Pt. C, No. 3, May 1982, pages 111-118. It is clear from this publication how the symmetrical components can be described as a phase-rotating operator, that is, with a certain amplitude and phase angle or as a complex quantity with real and imaginary parts.